The Ethernet LAN Service defined by the Metro Ethernet Forum (MEF) is likely to become an important Layer 2 Virtual Private Network service, being capable of securely interconnecting selected Ethernet LANs via one or more provider transport networks. The service must emulate the legacy Ethernet LAN operation from the point of view of a customer of a transport network operator. Therefore, both unicast and broadcast forwarding of the client Ethernet frames must be supported across the transport network(s).
FIG. 1 illustrates schematically an example provider's transport network comprising a multiplicity of Provider Edge (PE) nodes and internal P routers, interconnected by trunks (e.g. optical fibres) of the transport network. A plurality of corporate Ethernet LANs are connected to the transport network via respective Customer Edge (CE) nodes that are in turn connected to corresponding PE nodes. The Virtual Private LAN Service (VPLS) [1] is an Ethernet LAN service provided over IP/MPLS-based networks and applies the Pseudo-wire End-to-End (PWE) architecture. Full mesh connectivity between the provider edge nodes (PE) is established with point-to-point connections. PE nodes implement a virtual bridge emulating the MAC learning functionality. However, until a PE has received a frame from a given MAC address, it does not know over which port that particular address is reachable. Therefore, if a PE receives an Ethernet frame with a previously unseen destination address, it will send the frame to all other PEs within the appropriate customer service set (i.e. to those PEs connected to Ethernet LANs belonging to the same Wide Area Network (WAN) as the LAN from which the frame originates).
According to VPLS, the ingress PE replicates the frame and sends one copy to each remote PEs over point-to-point pseudo-wires. In order to decrease the bandwidth consumed by this ingress based frame replication, a set of multicast trees can be deployed in addition to the full mesh point-to-point connectivity as described in [2]. Consider for example a given customer WAN comprising four Ethernet LANs, with each LAN being coupled to a CE and in turn to a PE. A multicast tree is established for each of the four PEs (by appropriately configuring forwarding tables in the PEs and the intervening routers), such that a frame received at an ingress PE is forwarded up the tree towards the three other PEs. Branching of frames occurs at intervening routers. Nonetheless, duplication of frame sending is significantly reduced. Currently RSVP-TE supports the establishment of point-to-point [6], point-to-multipoint [7], and multipoint-to-point [8] connections over MPLS.
Ethernet standards are being amended to equip Ethernet with new features in support of Carrier Ethernet capabilities. Provider Bridging (PB) [3] and Provider Backbone Bridging (PBB) [4] are enhancing Ethernet scalability, and may even replace MPLS in future transport networks. With PB, a new VLAN tag, Service VLAN (S-VLAN), is introduced to allow providers to use a separate VLAN space while transparently maintaining the customer VLAN (C-VLAN) information. PBB allows for full separation of the customer and provider address spaces by encapsulating customer frames with the addition of a “backbone” MAC header. This allows both the MAC addresses and the whole VLAN space to be under the control of the provider. PBB-TE [5] decouples the Ethernet data and control planes by explicitly supporting external control/management mechanisms to configure static filtering entries in bridges and create explicitly routed connections.
Generalized Multi-protocol Label Switching (GMPLS) is a candidate control plane for PBB-TE and indeed the IETF is currently specifying GMPLS extensions for PBB-TE. GMPLS is a general control plane architecture for different Layer 1 and Layer 2 forwarding technologies. GMPLS uses specific protocols to support the dissemination of the data plane parameters (routing protocols with Traffic Engineering extension: OSPF-TE/ISIS-TE) and the establishment of connections between nodes (signaling protocols: RSVP-TE). As with MPLS, RSVP-TE will allow the establishment of multicast trees within the PBB-TE based transport network to facilitate Ethernet frame broadcast simulation.
Whether in MPLS or PBB-TE based transport networks, simulating Ethernet frame broadcasting using a set of multicast trees is an expensive function to manage. For example, for a WAN involving twenty PE nodes, as well as point-to-point connections between each and every PE, twenty multicast trees are required.